The Parkes survey of 21-Centimeter absorption in discrete-source spectra. II. Galactic 21-Centimeter observations in the direction of 35 extragalactic sources

Abstract

A detailed comparison of emission and absorption spectra obtained in the direction of extragalactic sources permits an unambiguous separation of the contribution from a diffuse, high-temperature, optically thin component and that from denser and colder local concentration of hydrogen. The concentrations have a mean column density N_Hc≈ 3 × 10²⁰ atoms cm^-2 and a spin temperature T_s ≈ 60°-80° K. They are typically separated along a line of sight by one full scale-thickness of the galactic disk. The diffuse intercloud medium has a column density N_Hi ≈ 1.4 × 10²⁰ cosec |b| atoms cm^-2, a spin temperature in excess of 750° K, and contains half of the total mass of hydrogen in the solar neighborhood.